Recent experimental evidence has shown that the Snellen acuity test which is popularly used for testing vision is inadequate for identifying certain abnormalities in an individual's vision. Although the Snellen test can reveal what size letter a person can recognize, it has no means for measuring the level of contrast necessary for letter recognition.
Contrast can be qualitatively equated to the degree of sharpness in a photocopy of a document. In a high quality photocopy (high contrast) the black letters stand out from the white background. In a poor quality photocopy (low contrast) the dark letters are barely discernible over the light gray background.
Reduced contrast sensitivity, due to abnormal spatial frequency channels, may explain vision dimness and other vision abnormalities. There is firm evidence that certain patients suffering from multiple sclerosis complain about the quality of vision in one eye compared to another when both eyes test as having normal Snellen acuity. However, when contrast sensitivity is measured, the reduced sensitivity explains why an otherwise normal eye has impaired vision. This suggests that contrast sensitivity measurements may be an early diagnostic aid in some patients suspected to have multiple sclerosis.
Abnormal spatial frequency channels and consequent reduced contrast sensitivity may explain some defects in perception including form perception.
In a study of 36 patients with cerebral lesions who complained of blurred vision, yet had 20/40 acuity or better, it was shown that the blurred vision could be explained using contrast sensitivity testing. These deficiencies could be grouped into three broad categories of loss: high frequency loss, loss at all frequencies and loss at a particular frequency.
Glaucoma patients exhibit a different contrast sensitivity defect due to glaucomatous visual damage which effects spatial frequency filtering. Thus contrast sensitivity testing could be of clinical significance concerning early therapy for glaucoma-prone eyes.
Corneal edema is a problem associated with wearing contact lenses. Contrast sensitivity measurements could provide important clinical help in evaluating the effectiveness and safety of soft contact lenses.
Some cataract patients with normal or near-normal acuity reveal a considerable low frequency deficit in contrast sensitivity while sensitivity to high spatial frequencies remains intact. Another group of cataract patients can be identified by a contrast level loss. So contrast sensitivity testing may be useful in the presurgical evaluation of these patients.
Patients suffering from retinitis pigmentosa may have a spatial frequency deficiencies much worse than Snellen acuity reveals. Such abnormalities have also been reported in several forms of macular dystrophies, also not revealed by Snellen testing.
It is also claimed that contrast sensitivity measurements have: detected early changes in the crystalline lens; predicted the mobility of low vision patients; quantified the vision of amblyopes; assessed performance changes with visual training; reflected subtle residue of acute optic neuritis; and defined visual losses from exposure to toxic substances.